Make sure your volume is up, only have one tab with Hopin audio open, try using headphones, and if you change audio/camera settings or headphone connectivity you need to refresh your browser.
Even though Hopin is cloud-based, if you're on a PC use Windows 10. Mac users should be okay. Read more for full operating system troubleshooting.
Chrome is preferred, with Firefox also recommended. Find troubleshooting tips below.